The invention relates to all-or-nothing control circuits for the passage of the current in a load of the type utilising at least one principal switching transistor unblocked by injection of a substantially constant current into its base under the action of a control signal.
Among the applications of such circuits there can be mentioned the switching of rest and working signals in a proximity detector or inductive presence detector or a detector with photo-electric cell fed with continuous current.
The problem that the present invention proposes to resolve is that of the protection of the principal switching transistor against overloads or short-circuits at the terminals of the load.
In a known solution, this problem is resolved by having the circuit comprise means for comparing the collector-emitter voltage of the principal transistor with a reference voltage and for blocking the said transistor as soon as the said collector-emitter voltage exceeds the said reference voltage.
In the manner of construction described in French Patent No. 7613927 filed on May 10, 1976 in the name of the same Assignee the said means comprise a first auxiliary transistor, the emitters of the two transistors being common, the collector of the principal transistor and the base of the auxiliary transistor being coupled to a common point, respectively by means of two diodes connected head to tail and a second auxiliary transistor, connected with the first as an unstable multi-vibrator, the collector of the first auxiliary transistor being coupled to the base of the principal transistor.
So long as the voltage of the collector-emitter of the principal switching transistor remains lower than the maximum permitted value, the first auxiliary transistor is blocked and the second is then in the conductive state. Above this maximum value, the first auxiliary transistor becomes unblocked, which has the result of blocking the principal switching transistor. Furthermore, the unblocking of the first auxiliary transistor has the result of blocking the second, thus causing the unstable multi-vibrator to pass to its other state, which provides a pulse on the base of the principal switching transistor, which again passes to the saturated state and can remain therein, if the load has resumed its normal value. There is thus automatic re-setting of the circuit after the disappearance of the short-circuit.